U.S. patent application number 10/552539 was filed with the patent office on 2006-09-28 for method for the production of a blank mold for optical fibers.
This patent application is currently assigned to Heraeus Tenevo GmbH. Invention is credited to Karsten Brauer, Heinz Fabian, Michael Hunermann, Richard Schmidt, Gerhard Schotz, Norbert Treber.
Application Number | 20060213228 10/552539 |
Document ID | / |
Family ID | 33103306 |
Filed Date | 2006-09-28 |
United States Patent
Application |
20060213228 |
Kind Code |
A1 |
Schotz; Gerhard ; et
al. |
September 28, 2006 |
Method for the production of a blank mold for optical fibers
Abstract
In a known method for the production of a blank mold for optical
fibers, a fluorine-doped SiO.sub.2 enveloping glass is produced on
a core glass cylinder that rotates about its longitudinal axis,
wherein a silicon-containing starting substance is fed to a plasma
burner, said substance is then oxidized in a plasma flame assigned
to the plasma burner to obtain SiO.sub.2 particles, the SiO.sub.2
particles are deposited by layers on the enveloping surface of the
cylinder of the core glass cylinder in the presence of fluorine and
sintered into the enveloping glass. The invention aims at providing
an economical method, which builds upon the above-mentioned method,
in order to produce a blank mold from which optical multi-mode
fibers (52) can be obtained. In comparison with fibers (51)
produced according to standard methods, said optical multi-mode
fibers are characterized by high initial transmission in the UV
wavelength range and good resistance with respect to brief UV
radiation, more particularly in the 210-300 nm wavelength range.
According to the invention, a plasma flame that irradiates an
ultraviolet light having a wavelength of 214 nm with an intensity
of at least 0.9 ?W--determined on the basis of plasma flame
intensity measurement--is used for the formation and deposition of
the SiO.sub.2 particles on the core glass.
Inventors: |
Schotz; Gerhard;
(Aschaffenburg, DE) ; Brauer; Karsten;
(Bruchkobel, DE) ; Hunermann; Michael; (Alzenau,
DE) ; Schmidt; Richard; (Hammersbach, DE) ;
Fabian; Heinz; (Grossostheim, DE) ; Treber;
Norbert; (Hanau, DE) |
Correspondence
Address: |
TIAJOLOFF & KELLY
CHRYSLER BUILDING, 37TH FLOOR
405 LEXINGTON AVENUE
NEW YORK
NY
10174
US
|
Assignee: |
Heraeus Tenevo GmbH
Quarzstrasse 8
Hanau
DE
63450
|
Family ID: |
33103306 |
Appl. No.: |
10/552539 |
Filed: |
April 6, 2004 |
PCT Filed: |
April 6, 2004 |
PCT NO: |
PCT/EP04/03665 |
371 Date: |
October 11, 2005 |
Current U.S.
Class: |
65/377 ; 65/378;
65/391; 65/421 |
Current CPC
Class: |
C03B 37/01291 20130101;
C03B 37/01466 20130101; Y02P 40/57 20151101; C03B 2201/12 20130101;
C03B 37/01211 20130101; C03B 37/01426 20130101 |
Class at
Publication: |
065/377 ;
065/391; 065/378; 065/421 |
International
Class: |
C03B 37/07 20060101
C03B037/07; C03B 37/018 20060101 C03B037/018 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 9, 2003 |
DE |
103 16 487.1 |
Claims
1. A method for producing a preform for optical fibers, said method
comprising: producing a fluorine-doped SiO.sub.2 cladding glass on
a core glass cylinder rotating about a longitudinal axis thereof,
including feeding a plasma burner with a silicon-containing starter
substance, said starter substance being oxidized in a plasma flame
of the plasma burner to obtain SiO.sub.2 particles, and depositing
the SiO.sub.2 particles in layers on a cylindrical outer surface of
the core glass cylinder in the presence of fluorine and sintering
said SiO.sub.2 particles deposited into the cladding glass, wherein
said plasma flame emits ultraviolet light in one or more
wavelengths in a range about a wavelength of 214 nm at an intensity
of at least 0.9 .mu.W, determined on the basis of a plasma flame
intensity measurement, during the forming and depositing of the
SiO.sub.2 particles on the core glass cylinder.
2. The method according to claim 1, wherein the plasma flame emits
said ultraviolet light at an intensity ranging from 1.0 .mu.W to
1.4 .mu.W.
3. The method according to claim 1, wherein the cylindrical outer
surface of the core glass cylinder is kept at a surface temperature
ranging from 1550.degree. C. to 2000.degree. C. during deposition
of the SiO.sub.2 particles, and wherein the core glass cylinder has
an outer diameter of at least 40 mm.
4. The method according to claim 3, wherein the cylindrical outer
surface of the core glass cylinder is kept at a surface temperature
ranging from 1700.degree. C. to 1900.degree. C. during deposition
of the SiO.sub.2 particles, and wherein the core glass cylinder has
an outer diameter of at least 60 mm.
5. The method according to claim 1, wherein the SiO.sub.2 particles
are deposited on the cylindrical outer surface in layers having a
layer thickness such that optical fibers derived from the preform
have optical fiber layers yielded by the layers of the deposited
SiO.sub.2 particles, and said optical fiber layers have respective
layer thicknesses of not more than 0.05 .mu.m in the optical
fibers.
6. A method for producing a preform for optical fibers said method
comprising: providing a cladding glass tube of fluorine-doped
quartz glass for cladding a core glass cylinder, wherein said
cladding glass tube is produced by supplying a silicon-containing
starter substance to a plasma burner, said plasma burner having a
plasma flame that oxidizes said substance forming SiO.sub.2
particles, said SiO.sub.2 particles being deposited in layers in
the presence of fluorine on a cylindrical outer surface of a
substrate tube which is rotating about a longitudinal axis thereof
and is made from quartz glass, and wherein said particles are
sintered, collapsing said cladding glass tube onto said core glass
cylinder, and removing said substrate tube prior to collapsing said
cladding glass tube.
7. The method according to claim 6, wherein said substrate tube is
removed by etching.
8. The method according to claim 6, wherein said substrate tube is
etched off during production of the cladding glass tube by
introducing an etching gas.
9. The method according to claim 8, wherein said etching gas is
SF.sub.6.
10. The method according to claim 6, wherein said substrate tube
has a wall thickness ranging from 2 mm to 10 mm.
Description
[0001] The present invention relates to a method for producing a
preform for optical fibers by producing a fluorine-doped SiO.sub.2
cladding glass layer on a core glass cylinder rotating about its
longitudinal axis in that a plasma burner is fed with a
silicon-containing starter substance, said substance is oxidized in
a plasma flame assigned to the plasma burner to obtain SiO.sub.2
particles and the SiO.sub.2 particles are deposited in layers on
the cylindrical outer surface of the core glass cylinder in the
presence of fluorine and are sintered into the cladding glass.
[0002] Such a method for producing a preform for optical fibers, as
well as a generic preform, are described in DE 25 36 457 A1. For
producing the preform a core glass cylinder of undoped quartz glass
is provided, and fluorine-doped quartz glass is deposited on the
cylindrical outer surface thereof as a cladding glass layer. For
producing a cladding glass layer an induction-coupled plasma burner
is used and fed with a gas stream containing a hydrogen-free
silicon compound and oxygen. Moreover, a fluorine-containing
compound is introduced into the plasma flame assigned to the plasma
burner. Fluorine-containing SiO.sub.2 particles are formed from the
starter substances in the plasma flame. These are deposited in
layers onto the core glass cylinder rotating about its longitudinal
axis and are directly sintered onto the core glass layer with
formation of a fluorine-containing SiO.sub.2 cladding glass
layer.
[0003] The described method for producing a preform for optical
fibers is also called "POD (plasma outside deposition) method". The
core glass cylinder is normally produced by oxidation or by flame
hydrolysis of silicon-containing starter substances by means of
methods which are generally known under the names VAD (vapor phase
axial deposition) method, OVD (outside vapor phase deposition)
method, MCVD (modified chemical vapor deposition) method and PCVD
or also PECVD (plasma enhanced chemical vapor deposition) method.
The core glass cylinder consists of undoped quartz glass most of
the time, but may also contain dopants that change the refractive
index.
[0004] The present invention also refers to a method for producing
a preform for optical fibers by providing a cladding glass tube of
fluorine-doped quartz glass for over-cladding a core glass, with a
silicon-containing starter substance being supplied to a plasma
burner for producing the cladding glass tube, said substance being
oxidized in a plasma flame assigned to the plasma burner to obtain
SiO.sub.2 particles and the SiO.sub.2 particles being deposited in
layers in the presence of fluorine on the cylindrical outer surface
of a substrate tube of quartz glass rotating about its longitudinal
axis, and said particles being sintered.
[0005] Such a method is described in U.S. Pat. No. 6,253,580 B1.
For producing a cladding glass tube of fluorine-doped quartz glass
according to the POD method, a dry plasma flame is produced in
which SiCl.sub.4 is oxidized into SiO.sub.2 particles, and said
particles are deposited and immediately vitrified on a substrate
tube. A cladding glass tube of fluorine-doped quartz glass is
obtained by introducing fluorine into the plasma flame. The
substrate tube consists of doped or undoped quartz glass. The
resulting cladding glass tube is used as a cladding material for a
core glass for producing a preform for optical fibers.
[0006] Optical fibers are obtained by elongating the preform in a
drawing method. They are inter alia used for the transmission of
high-energy ultraviolet radiation, for instance for spectroscopic,
medicinal or photolithographic applications for producing
semiconductor components. The corresponding apparatus and machines
are often equipped with excimer lasers emitting high-energy pulsed
laser radiation of a wavelength of 248 nm (KrF laser) or of 193 nm
(ArF laser).
[0007] Short-wave UV radiation in the wavelength range between 190
nm and 250 nm may create absorption-causing defects in the quartz
glass of the fibers. Various damage mechanisms and their progress
have been described. The quartz glass fibers often show a gradual
decrease in transmission at the beginning of irradiation. The
transmission decreases to a "plateau value" from which it hardly
changes even during prolonged irradiation. This effect is called
"photodegradation". The extent of defect formation and
photodegradation depends on the quality of the respective quartz
glass, the quality being essentially defined by structural
properties such as density, refractive index profile, homogeneity
and chemical composition. An important parameter is here the number
of so-called "precursor centers". These are understood as defects
that exist in the quartz glass matrix of the preform or the fiber
already at the beginning and lead to a rise in absorption during
continued UV irradiation (induced absorption). It has been found
that in the known method, presumably due to the UV portion of the
plasma flame, an induced absorption is created in the UV wavelength
range that may lead to a low initial transmission of the optical
fiber, and that "precursor centers" are also produced in large
numbers in the core glass cylinder during deposition of the
cladding glass layer, such centers leading to an intensified
photodegradation in the fiber. Therefore, the preforms produced
according to the known method often lead to an unfavorable behavior
of the fibers drawn therefrom with respect to short-wave UV
radiation.
[0008] It is the object of the present invention to provide an
economic method for producing a preform from which fibers can be
obtained having a high initial transmission in the UV wavelength
range and a high resistance to short-wave UV radiation.
[0009] Starting from the above-mentioned method this object is
achieved according to the invention in that a plasma flame which
emits ultraviolet light of a wavelength of 214 nm with an intensity
of at least 0.9 .mu.W, determined on the basis of the plasma flame
intensity measurement, is used for forming and depositing the
SiO.sub.2 particles on the core glass cylinder.
[0010] In a first variant of the method according to the invention,
a plasma flame is produced for forming and depositing the SiO.sub.2
particles on the core glass cylinder, the plasma flame emitting a
minimum intensity of ultraviolet radiation of 0.9 .mu.W at the
wavelength of 214 nm. A suitable method for measuring the plasma
flame intensity in the ultraviolet wavelength range has not been
described yet in the technical literature. To be able to
quantitatively determine the UV radiation emitted by the plasma
flame, a method was therefore developed whose measuring set-up and
measuring conditions will be explained further below with reference
to FIG. 3. This measuring method will also be called PFI (plasma
flame intensity measurement) method in the following.
[0011] As expected, it was found on the basis of such plasma flame
intensity measurements that the emitted UV intensity increases with
the electrical power fed into the plasma. In the former POD
methods, the intensity of the UV radiation of said wavelength was
normally about 0.8 .mu.W. It was found that an increase in the
intensity of shortwave UV radiation of the plasma flame yields
preforms from which optical fibers are drawn that are characterized
by a higher initial transmission in the UV range and a better
damage behavior in comparison with high-energy UV radiation.
[0012] One possible explanation for this surprising effect is that
due to the high UV intensity at the wavelength of 214 nm a defect
formation is started that decreases from the surface of the core
glass cylinder to the inside. The defects concerned are defects
producing absorption in the UV range (induced absorption), and also
"precursor defects". These defects, in turn, cause an absorption
having a maximum which is also in the range of the wavelength of
214 nm, said absorption being particularly efficient in the
near-surface region. The "intrinsic absorption" of the core glass
cylinder produced in this way therefore reduces the further action
of high-energy UV light of the plasma flame, so that the
penetration of the damaging UV radiation into central layers of the
core glass cylinder where the formation of defects and precursor
defects is particularly critical is reduced. It is important that
the core glass cylinder is shielded by this "intrinsic absorption"
particularly of UV radiation of the wavelength range between 190 nm
and 240 nm, for this radiation is decisive for the initiation of
defects with UV absorption bands and their precursor defects in the
core glass cylinder. UV radiation with a wavelength of less than
about 190 nm is mainly kept away from the core glass cylinder by
the air or intrinsic absorption of the plasma flame, while the UV
radiation portion of the plasma flame of a comparatively long-wave
range above 240 nm does not produce any significant defects in the
quartz glass.
[0013] Hence, in the method of the invention the further action of
harmful UV radiation of the plasma flame on the core glass cylinder
is reduced by the rapid generation of UV-radiation absorbing
defects. Hence, in the final analysis the damage caused by the
great intensity of the plasma flame on the surface of the core
glass cylinder does not effect an increase, but surprisingly a
decrease in the mean effective damage dose per volume of core glass
cylinder material in comparison with the prior-art procedure. The
phase of the deposition process is here decisive in which no
cladding glass layer or only a thin one is present on the core
glass cylinder. In a later phase of the deposition process, the UV
intensity of the plasma flame can also be reduced again. The plasma
is produced by means of high-frequency alternating current. The UV
intensity of the plasma flame at the wavelength 214 nm is
determined by the electrical power introduced into the plasma. The
higher this power, the greater is in general the UV intensity of
the plasma flame, unless counteracting measures are taken, such as
the introduction of UV-absorbing gas into the plasma flame. The
method of the invention has turned out to be particularly
advantageous in preforms from which optical fibers are produced
that are intended to be used in combination with ultraviolet light
of a short wavelength. The preform produced in this way shows a
comparatively small number of defects and precursor defects in the
center, so that both a high initial transmission in the UV range
and a low induced attenuation are observed in the use of a fiber
obtained from a preform produced according to the invention in
combination with excimer radiation of 248 nm and 193 nm.
[0014] This result was found for a specific distance range between
the surface of the developing preform (core glass cylinder and
cladding glass deposited thereon) and the plasma flame. It is
assumed that said distance has little influence on the
defect-producing effect of the UV radiation, so that similar or
slightly deviating results might be obtained at other distances.
The plasma flame is produced inside a reaction sleeve which is
surrounded by a high-frequency coil. This coil defines the
excitation range for the plasma when the visible region of the
plasma flame can also project beyond the end of the high-frequency
coil. For definitely indicating the distance between the surface of
the developing preform and the plasma flame the end of the
high-frequency coil which faces the preform is defined as the place
of the plasma flame. Measured from said place, this yields the
distance range set in practice between the surface of the
developing preform and the plasma flame between 60 mm and 90
mm.
[0015] In a preferred procedure, a plasma flame is used that emits
light of a wavelength of 214 nm with an intensity in the range of
1.0 .mu.W to 1.4 .mu.W.
[0016] UV radiation of 214 nm above the indicated lower intensity
limit effects a particularly rapid formation of the UV-radiation
absorbing damage which reduces the further action of the UV
radiation of the plasma flame and the accompanying defect formation
in the center of the core glass cylinder. With an intensity above
the said upper limit, the extent of the damage caused in the edge
portion outweighs the absorbing and shielding effect thereof.
[0017] It has also turned out to be advantageous to keep the
cylindrical outer surface of the core glass cylinder during
deposition of SiO.sub.2 at a surface temperature ranging from
1550.degree. C. to 2000.degree. C., preferably from 1700.degree. C.
to 1900.degree. C., with the proviso that the core glass cylinder
has an outer diameter of at least 40 mm, preferably of at least 60
mm.
[0018] In the deposition process, the core glass cylinder is heated
up, which has the effect that diffusion processes take place more
rapidly. As a result, impurities may e.g. pass into the center of
the core glass cylinder more easily, or predetermined concentration
profiles of a dopant distribution may be impaired. It is therefore
desirable to keep the heating up of the core glass cylinder during
deposition as small as possible. On the other hand, a certain
heating up is indispensable for depositing and sintering the
cladding glass layer. The temperature increases from the
cylindrical outer surface of the core glass cylinder towards the
inside. Thus, a lower temperature prevails in the interior of the
core glass cylinder than in the area of the cylindrical outer
surface. When a core glass cylinder is used having an outer
diameter of at least 40 mm, preferably at least 60 mm, and in
combination with the said surface temperature which is measured at
the point of impingement of the plasma flame on the surface of the
core glass cylinder (or at the point of impingement of the
extension of the main propagation direction of the plasma flame
towards the surface), a temperature evidently prevails in the
center of the core glass cylinder that is so low that diffusion
processes are hardly noticed. This improves the purity and
reproducibility of the fiber properties and the values set for the
refractive index profile and the attenuation characteristics of the
core glass cylinder can be observed more easily.
[0019] With respect to a high transmission it has further turned
out to be of advantage when the SiO.sub.2 particles are deposited
on the cylindrical outer surface in layers at a layer thickness in
such a manner that they yield layers with thicknesses of not more
than 0.05 .mu.m in the optical fiber.
[0020] In dependence upon the draw ratio between preform and fiber,
the formation of thin layers on the cylindrical outer surface has
the effect that said layers are present in the optical fiber drawn
from the preform in layers having thicknesses of less than 0.05
.mu.m. Said layer thickness is clearly below the wavelength of the
light guided in the fiber, whereby interactions between the
individual layers and the light are avoided. By contrast, layers of
a greater thickness which in the optical fiber lead to layers
having a thickness of more than 0.1 and are thus in the order of
the wavelength of the light guided therein impair optical
transmission. The core diameter of a typical multimode fiber is 200
.mu.m. When a core glass cylinder is used having an outer diameter
of 70 mm, this yields a draw ratio of 350, so that in this example
advantageous layer thicknesses of the cladding glass layer are less
than 15 .mu.m according to the invention.
[0021] Starting from the method described at the outside and used
for producing a preform using a cladding glass tube, the
above-indicated object is also achieved according to the invention
in that the cladding glass tube is collapsed onto the core glass
cylinder and that the substrate tube is removed prior to
collapsing.
[0022] In this variant of the method according to the invention, it
is not the core glass cylinder that is used as a carrier for the
POD method, but a substrate tube of quartz glass is employed. Due
to the UV portion of the plasma flame in the deposition process,
defects are bound to be formed in the quartz glass matrix of the
substrate tube, as has been explained above.
[0023] However, since the substrate tube is removed prior to
collapsing of the cladding glass tube, the damage to the substrate
tube has no effect on the core glass of the preform and the fiber
obtained therefrom. Thus, in this variant of the method the core
glass cylinder remains entirely unaffected by the UV radiation of
the plasma flame.
[0024] The UV-absorbing defects and the precursor defects produced
in the cladding glass due to the production process have only
little influence on the attenuation and the radiation resistance of
the fiber because, on the one hand, fluorine doping reduces defect
formation and, on the other hand, the intensity guided in the
cladding glass is low in a multimode fiber having a typical
numerical aperture (NA) of 0.22 and a core diameter of 200 .mu.m.
The resulting multimode fibers are characterized by low absorption
at a wavelength of 214 nm, which is distinctly less than 1 db/m, as
a rule even less than 0.7 db/m.
[0025] The substrate tube can be removed mechanically (by grinding,
polishing, drilling) or chemically (by etching). The last-mentioned
procedure has turned out to be particularly suited.
[0026] Since the substrate tube is removed by etching, impurities
created by mechanical tools or abrasives are prevented from
penetrating into the cladding glass layer deposited on the
substrate tube.
[0027] After completion of the POD method the substrate tube can be
removed. However, it has turned out to be particularly advantageous
when the substrate tube is etched off during formation of the
cladding glass tube by introducing an etching gas into the inner
bore thereof.
[0028] The etching off of the substrate tube in the POD method
shortens the process time on the whole, as compared with etching at
a later time, whereby the process costs are reduced and undesired
diffusion of fluorine due to a hot process is diminished. At the
same time, the etching gas may produce a predetermined internal
pressure for stabilizing the inner bore.
[0029] SF.sub.6 is preferably used as the etching gas.
[0030] SF.sub.6 effects a rapid etching off of the quartz glass
with formation of volatile compounds of silicon and fluorine and
counteracts a diffusion of fluorine out of the cladding glass tube
at the same time.
[0031] To ensure an adequate thermal loadability of the substrate
tube on the one hand and to facilitate the etching-off operation on
the other hand, a substrate tube is used that has a wall thickness
ranging from 2 mm to 10 mm.
[0032] In the preform obtained according to this method, no defects
or only a few defects caused by UV radiation are found in the
contact surface adjoining the inner cladding glass surface, so that
a multimode fiber drawn from said preform has an attenuation of not
more than 1 db/m at a wavelength of 214 nm.
[0033] The preform produced according to the invention is
particularly preferably used for producing fibers for the
transmission of UV radiation of a high energy density in the
wavelength range between 190 nm and 250 nm. Thanks to its high
transmission and its radiation resistance it is particularly well
suited for the transmission of high-energy excimer laser UV
radiation having wavelengths of 248 nm and 193 nm.
[0034] The invention shall now be explained in more detail with
reference to embodiments and a patent drawing. The patent drawing
shows in detail in
[0035] FIG. 1 the POD method for producing a preform, in a
schematic illustration;
[0036] FIG. 2 a radial cross-section through a preform produced
according to the method of the invention, in a schematic
illustration;
[0037] FIG. 3 the measuring set-up used for measuring the intensity
of the plasma flame in the UV wavelength range according to the PFI
method;
[0038] FIG. 4 a diagram for absorbing the damage caused in the core
glass cylinder by the UV portion of the plasma flame; and
[0039] FIG. 5 a diagram for optically attenuating different optical
fibers in the wavelength range between 200 nm and 350 nm.
[0040] FIG. 1 schematically illustrates the method for producing a
preform for so-called multimode fibers with a stepped refractive
index profile. To this end a rod 3 of high-purity undoped synthetic
quartz glass having a diameter of 86 mm is provided and coated by
means of a "plasma outside deposition (POD) method" with a cladding
4 of fluorine-doped quartz glass. To this end SiCl.sub.4, oxygen
and SF.sub.6 are supplied to a plasma burner 1 and converted in a
burner flame 2 assigned to the plasma burner 1 into SiO.sub.2
particles. The main propagation direction of the plasma flame 2 is
illustrated by a dotted line 5. In reversingly moving the plasma
burner 1 along the rod 3 from one end to the other end, the
SiO.sub.2 particles are deposited in layers on the cylindrical
outer surface of the rod 3, which is rotating about its
longitudinal axis 6. It is thereby possible to incorporate high
fluorine concentrations of more than 3% by wt. in the quartz glass
network of the cladding 4. The plasma flame 2 is produced inside a
reaction sleeve 8 of quartz glass which is surrounded by a
high-frequency coil 7. The high-frequency coil 7 has a height of
about 92 mm, and the reaction sleeve 8 projects beyond the coil by
about 7.5 mm. A distance of 65 mm, which does also not change in
the course of the deposition process, is set between the upper end
of the high-frequency coil 7 and the surface of the rod 3.
[0041] According to the invention the intensity of the plasma is
adjusted by supplying a corresponding electrical power in such a
manner that the plasma flame 2 emits UV radiation of a wavelength
of 214 nm with an intensity of 1.3 .mu.W. As a consequence, defects
and precursor defects which cause absorption in the wavelength
range between 190 nm and 250 nm and which will be explained in more
detail below with reference to FIGS. 4 and 5 are produced in the
rod 3, particularly in the near-surface regions of the rod 3.
[0042] In the area of the point of impingement of the plasma flame
2 on the surface of the rod 3 or the cladding 4, the surface
temperature is continuously measured by means of an IR camera. With
an increasing outer diameter of the developing preform, the surface
is increasing and the surface temperature is decreasing
accordingly. To maintain a constant temperature of 1800.degree. C.
in the area of the surface, the intensity of the plasma flame 2 is
continuously increased.
[0043] The rotational speed of the rod 3 and the translational
speed of the plasma burner 1 are adjusted such that the individual
cladding glass layers have a mean thickness of about 12 .mu.m. At a
draw ratio of preform and fiber this yields a multimode fiber
having a core diameter of 200 .mu.m and cladding glass layers
having thicknesses of about 0.03 .mu.m, which are clearly below the
operating wavelength in the intended use of the fibers and do thus
not impair the transmission properties thereof.
[0044] FIG. 2 shows the preform obtained according to the method in
a radial cross-section. Reference numeral 21 is assigned to the
preform for optical fibers on the whole. The preform consists of a
core 22 of pure quartz glass which has a refractive index of 1.4571
at 633 nm, and of a cladding 23 of fluorine-doped quartz glass
which has a refractive index of 1.440 at a wavelength of 633 nm.
The fluorine content of the cladding glass is 5% by wt. The content
of hydroxyl groups in core 22 is 700 wt ppm. The core 22 has a
diameter of 85 mm, and the cladding 23 has an outer diameter of
93.5 mm. In the area of the contact surface between core 22 and
cladding 23, a region which projects into the core 22 is
illustrated by dotted lines, said region pointing at a "damage
layer" 24 which is very strongly penetrated by structural defects.
The defects of the damage layer 24 are produced during the
deposition process due to the high UV intensity of the plasma flame
2. The density of said defects decreases inside the damage layer 24
and also inside the core 22 from the outside to the inside, so that
it is not possible to indicate an exact thickness.
[0045] The measuring method used for measuring the UV intensity of
the plasma flame, as well as the measuring set-up, shall now be
described with reference to FIG. 3:
[0046] In the PFI method used in this instance for determining the
plasma flame intensity, part of the flame center of the plasma
flame 2 is imaged onto a calibrated photodiode 31. With a CaF.sub.2
lens 32, which is spaced from the center of the plasma flame 2 at a
distance of 23 cm, an observation point is imaged from the flame
center onto a Polymicro UVMI fiber 33 having a length of 50 cm. The
front fiber end has a distance of 9.48 cm from the longitudinal
axis of the CaF.sub.2 lens 32. The light exiting at the other fiber
end of the UVMI fiber 33 is guided to the photodiode 31 after
having passed through a bandpass filter 34 which has a transmission
maximum at a wavelength of about 214 nm. The area of the photodiode
31 is here not completely illuminated. Important parameters of the
optical components for said measurement of the UV intensity of the
plasma flame are at a wavelength of 214 nm:
CaF.sub.2 Lens 32:
[0047] Focal length 94.8 mm; transmission 92.4%
UVMI fiber 33:
[0048] Length: 0.5 m; core diameter: 200 .mu.m; transmission 84.3%,
NA=0.22
Bandpass Filter 34:
[0049] Mid-wavelength: 214 nm; FWHM bandwidth: 10 nm, transmission:
17.3%
Photodiode 31:
[0050] Photocurrent/optical power: 0.77 mA/mW
[0051] The used UVMI fiber 33 is hydrogen-loaded and shows no
photodegradation in the measurement range of the UV radiation. The
photodegradation of the remaining components can be neglected.
[0052] The diameter D of the observation point results from the
numerical aperture (NA) of the fiber 33 and the focal length of the
used lens 32 as follows: D=2tan(arcsin(NA))9.48 cm=4.2 cm.
[0053] The viewing direction is not perpendicular to the vertically
extending main propagation direction 35 of the plasma flame 2, but
at an angle of about 82.degree. relative thereto, as shown by the
angular data on the horizontal 35. The region of maximum intensity
at the upper edge of the reaction sleeve 8 (and outside of said
sleeve) is chosen as the viewing point inside the plasma flame 2,
said sleeve projecting beyond the coil 7 by 5-10 mm. It can be
assumed that an even higher UV intensity could be measured inside
the reaction sleeve 8, which in practice can however only be
determined by taking great efforts. As a result, the measurement
yields an integral intensity over a portion of the plasma flame 2
that is illustrated in FIG. 3 as a projection 30 of the measuring
spot onto the envelope of the plasma flame 2.
[0054] The diagram of FIG. 4 schematically shows, over the
wavelength range between 180 nm and 360 nm, the relative
transmission (based on the initial transmission) of quartz glass
after damage caused by the UV radiation of the plasma flame 2. The
defects produced thereby create an absorption curve 41, of which a
pronounced absorption band is typical at a wavelength of 214 nm. It
could be demonstrated that the absorption band 41 mainly follows
from the superimposition of the absorptions of two types of
defects. The one type of defect has an absorption curve with a
pronounced absorption maximum at a wavelength of 214 nm (caused by
so-called E' centers; line 42) and the other type effects a flat
absorption curve (caused by so-called NBOH centers; line 43) in the
wavelength range of about 265 nm. On the whole, the defects
produced in this way subsequently create an absorption of the UV
radiation of the plasma flame 2 in the wavelength range between 180
nm and 260 nm and thus a decrease in the UV load on the center of
the core glass layer 22 (FIG. 2).
[0055] In the diagram of FIG. 5, the attenuation in dB/m as
measured on optical fibers is plotted on the y-axis, and the
wavelength on the x-axis. The upper one of the two illustrated
curves 51 shows the attenuation profile over the wavelength range
of 200 nm to 350 nm in the case of an optical fiber produced
according to the prior art, with an intensity of the plasma flame
of 0.7 .mu.m in the POD process. The curve 52 positioned thereunder
shows the attenuation profile in the case of an optical fiber which
has been produced from a preform produced according to the method
of the invention (with an intensity of the plasma flame of 1.2
.mu.m in the POD process).
[0056] It becomes apparent therefrom that the optical attenuation,
particularly at short wavelengths in the range between 210 nm and
300 nm, in the optical fiber 52 produced according to the method of
the invention is lower than in the fiber 51 produced according to
the standard method. The fiber 52 shows a distinctly lower
absorption particularly in the wavelength range around 215 nm and
265 nm and is therefore well suited for applications involving the
transmission of ultraviolet radiation, particularly the
transmission of high-energy UV radiation of a wavelength of 248 nm
and 193 nm.
[0057] As an alternative to the method explained with reference to
FIG. 1, the cladding glass layer is deposited by means of the POD
method on a substrate tube of quartz glass. The substrate tube has
an outer diameter of 86 mm and a wall thickness of 4 mm.
[0058] In the course of the deposition process, an etching gas
stream of SF.sub.6 is introduced into the bore of the substrate
tube. A cladding glass layer having a thickness of about 4.3 mm is
produced on the substrate tube, as described with reference to FIG.
1. The etching gas stream of SF.sub.6 is dimensioned such that the
substrate tube is completely removed directly before completion of
the outside deposition process, and it is only the cladding glass
tube having a wall thickness of about 4 mm that is obtained.
[0059] For producing a preform the cladding glass tube is collapsed
onto a core rod having a diameter of 85 mm. The preform is
characterized in that despite a cladding produced in the POD method
its core glass shows no defects that have been created by UV
radiation, e.g. of the plasma flame. A multimode fiber is drawn
from the preform with a core diameter of 200 .mu.m. At a wavelength
of 214 nm, said fiber shows an initial attenuation of 0.6 dB/m.
Moreover, the fiber was subjected to an UV irradiation test in
which a fiber having a length of 2 m and a core diameter of 200
.mu.m was irradiated by a deuterium lamp for four hours. The power
coupled into the fiber was here 70 nW/nm at the wavelength of 214
nm. Under these conditions an additional attenuation of 4 dB was
detected at 214 nm.
[0060] The fibers drawn from the preform are characterized by a
high transmission for UV radiation in the wavelength range between
190 nm and 250 nm and by a high UV radiation resistance.
* * * * *